Visual display systems are in widespread use. They are particularly valuable in the field of flight simulation. Flight simulation is a method of training aircraft personnel. In a simulator the trainee is placed in a realistically recreated aircraft environment designed such that the trainee, without leaving the ground, feels that he is in and operating an actual aircraft. An important element used to create this realism is the visual display system which provides the trainee with an out-of-the-window scene of the external environment.
The visual display system may include a front projection screen. This type of screen is used when a projector, and a trainee who observes the projected image, are located on the same side of the projection screen.
Screens for exhibiting projected images typically comprise a support structue plus some overlaying coating which comprises a screen surface. The present invention is directed to features of the screen surface.
A typical visual display system in flight simulation utilizes a front projection screen that comprises a segment of a spherical dome. The trainee, the projector, and the screen center of curvature, in the general case, are positioned at different interior locations of the dome. This displacement of the trainee's eye position from the location of the projector and the screen center of curvature produces a condition referred to as off axis viewing.
In order to provide the trainee with realistic visual cues, it is desirable to provide a projection screen surface which permits off-axis viewing with image brightness perceived by the trainee remaining constant over the whole screen surface.
Image brightness is a critical factor in the selection of a projection screen. Image brightness, or luminance, refers to a sensation in the consciousness of a human observer that corresponds to a visual sensation of more or less light i.e., bright, light, brilliant, dim, or dark. Image brightness is measured at the eye point.
One way to satisfy the condition that the trainee be provided with a screen surface which permits off-axis viewing with image brightness perceived by the trainee remaining constant over the whole screen surface, is to employ Lambertian surfaces. A Lambertian surface is a perfectly diffusing surface for which the image brightness or luminance is the same for every angle of observation. Therefore, the trainee perceives the same image brightness for any change in viewing angle defined by the trainee's eye line-of-sight.
Lambertian surfaces are perfectly diffuse and therefore reflect one hundred percent of the light incident on them. A unit, called the foot-lambert, is defined to take advantage of this circumstance. By definition, one foot-lambert is the brightness of a perfectly diffuse, perfectly reflecting surface illuminated by one foot-candle. Equivalently, a Lambertian surface is said to have a gain of one, since the brightness in foot-lamberts is numerically equal to the value of the illumination in foot-candles.
The gain of any screen surface is a measure of the magnitude of the image brightness. Gain magnitudes comprise a gain magnitude of one, for Lambertian surfaces which are perceived as white, up to infinity for other non-Lambertian surfaces. For non-Lambertian surfaces of gains greater than one, the gain is a measure of the image brightness of the screen surface measured along a normal to the surface. The eye is sensitive to changes in gain that correspond to absolute differences in gain of one or more. For example, the eye can perceive a change in image brightness that corresponds to a first screen surface with a gain of e.g., 2.5 compared to a second screen surface with a gain of 3.5.
Lambertian surfaces help to provide realistic visual cues to a trainee, since they enable the trainee to perceive a constant image brightness, with a gain of one, for every angle of observation. Lambertian surfaces thus provide constant image brightness for the case where the visual display system requires off-axis viewing and a gain of one. Nevertheless, in order to increase the realism of the visual display system, it is desirable to increase the magnitude of the luminance or image brightness. An increase in realism may be achieved by increasing the gain (which is a measure of image brightness along the normal to the screen surface).
A problem is presented, however, if the gain of the screen is fixed at a value that is not one. A screen surface that does not have a gain of one is not a Lambertian surface. For non-Lambertian surfaces the perceived brightness depends on the angle of observation or viewing angle. Since the perceived brightness depends on the viewing angle, the trainee subjectively believes that the fixed gain, which is measured from the normal to the surface, changes in correspondence to changes in the viewing angle. For example, a non-Lambertian surface of fixed gain four (4) as measured from the normal, corresponds to image brightness at the eye point for a viewing angle of e.g., 10 degrees, as an equivalent gain of only 2.5. Thus, with a non-Lambertian surface of fixed gain as measured from the normal, a change in viewing angle results in image brightness measured at the eye point that corresponds to only an equivalent gain. In general, as the viewing angle increases, the equivalent gain decreases and hence there is perceived diminution of image brightness.
Desirable visual cues correspond to the trainee's perception of constant image brightness, independent of changes of the viewing angle. The situation of diminished brightness as a function of viewing angle therefore critically detracts from the realism of the visual display system.
This problem has not been adequately addressed by the prior art. Much attention has been focused on using Lambertian surfaces. These surfaces are satisfactory for off-axis viewing, insofar as they provide image brightness with a gain of one. However, Lambertian surfaces do not have a sufficiently high gain for many applications. Surfaces which do have a sufficiently high fixed gain are non-Lambertian. These surfaces only provide an equivalent gain at eye point. The equivalent gain, moreover, changes from the fixed gain, as measured from the screen surface normal, according to changes in the viewing angle.
What is required in a flight simulation visual display system, where there is off-axis viewing, is a screen surface of gain greater than one; for this surface, constant image brightness at eye point should be maintained independent of changes in the viewing angle.